RPC_E_SERVERFAULT when calling Visual FoxPro COM object (out-of-process) from .Net?

I need to call a Foxpro (VFP 8) COM object from C#

The vfp code looks like this:

Define Class x2 As session OlePublic
    Function testInteropServer()
    ENDFUNC
Enddefine

The C# code looks like this:

[TestFixture]
public class TestFixture
{
    [Test]
    public void TestXtmBase()
    {
        xtmbase.x2 xtmBaseX2 = new xtmbase.x2();
        xtmBaseX2.testInteropServer();
    }
}

The COM server is compiled as an executable (not a dll). I can load it from Fox. It loads the object into .Net but I can't seem to call any of it's methods. I am importing the COM reference through the GUI in VS 2005. It recognizes all the methods and properties of the object. I just can't access them. I am getting the following error:

Test 'TestProject/TestFixture/TestXtmBase' failed:
    Execute
    System.Runtime.InteropServices.COMException: The server threw an exception.
        (Exception from HRESULT: 0x80010105 (RPC_E_SERVERFAULT))
    ErrorCode: -2147417851
    at xtmbase.x2Class.testInteropServer()

To remove the possibility of it being related to a COM executable I created a MT dll with the following code:

Define Class MTx2 As session OlePublic
  Function testInteropServer()
  ENDFUNC
Enddefine

I then created a console app:

class Program
{
    static void Main(string[] args)
    {
        mtx2.mtx2 mtx2 = new mtx2.mtx2();
        mtx2.testInteropServer();
        Console.WriteLine("Done");
    }
}

and it fails:

System.Runtime.InteropServices.COMException was unhandled
Message="The server threw an exception. (Exception from HRESULT: 0x80010105 (RPC_E_SERVERFAULT))"
Source="Interop.mtx2"
ErrorCode=-2147417851
StackTrace:
   at mtx2.mtx2Class.testInteropServer()
   at ConsoleTest.Program.Main(String[] args) in E:\development\iisx2\ConsoleTest\Program.cs:line 12
   at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

Any ideas why?

You are looking for the problem in the wrong process. It was the COM server that fell over on an exception. The RPC_E_SERVERFAULT error is merely a notification of that, a pretty useless one since all info about the exception was lost.

Attach a debugger to the server process before running the statement and have it stop on all exceptions so you can diagnose this. In VS that's done with Debug + Exceptions, tick the Thrown box for "Win32 Exceptions". Tools + Attach to Process.

The default handling is of course incredibly unhelpful. You can fix this in Vista and up with the IGlobalOptions interface. Use COMGLB_EXCEPTION_HANDLING to force the server to crash. This needs to be done in the server code, ought to be a bit of a problem in Foxpro I imagine.